Optical scanner and image forming apparatus

ABSTRACT

An optical scanner includes a light source, an optical coupler, an optical line image unit, a deflector, and an optical scanning unit. The optical scanning unit includes scanning lenses that guide the beams to a surface to be scanned. A surface on the deflector side of the scanning lens closest to a deflection reflecting surface has a negative power in a vertical scanning direction, and is a special toric surface in which a radius of curvature in a vertical scanning changes from an optical axis of the lens surface toward a periphery of the horizontal scanning direction. An F number of the beams toward the surface to be scanned of the scanning lens in the vertical scanning direction is larger in a peripheral part than in a central part in an effective scanning width.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present document is a divisional of U.S. application Ser. No.10/866,043 filed on Jun. 14, 2004 now U.S. Pat. No. 7,106,483, and thepresent document incorporates by reference the entire contents of U.S.application Ser. No. 10/866,043.

Further, the present document incorporates by reference the entirecontents of Japanese priority document, 2003-167508 filed in Japan onJun. 12, 2003.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to an optical scanner and an image formingapparatus that can be used in a laser beam printer (LBP), a digitalcopier, a plain paper facsimile (PPF), and the like.

2) Description of the Related Art

In recent electrographic image forming apparatus such as a laserprinter, densification of the image formed is speedy. To realizedensification of the formed image, it is necessary to realize a smallbeam spot diameter for optical scanning on an image forming surface ofan image carrier such as a photoconductor. The demand for such anoptical scanner is increasing.

(1) To achieve a small beam spot diameter, Japanese Patent ApplicationLaid-Open No. 2001-324689 proposes a two-lens scanning lens, with aspecial toric surface in which the radius of curvature in a verticalscanning section changes asymmetrically, toward the periphery of thehorizontal scanning direction, from the optical axis of the lenssurface, and the whole surface of the scanning lens is formed of thespecial toric surface.

(2) Further, Japanese Patent Application Laid-Open No. 2001-21824proposes an optical scanner to realize a stable and favorable opticalspot by correcting the wavefront aberration, where at least one surfaceof a lens included in a scanning imaging optical system is a verticalnon-arc plane such that the shape in the horizontal scanning section isan arc or non-arc, and the shape in the vertical scanning section isnon-arc, and the vertical non-arc plane is formed such that an incidentangle of the principal ray in deflected beams entering into therespective lens surfaces of the lenses in the scanning imaging opticalsystem, with respect to the normal on the lens surface, is 25 degrees orless in the whole area of the lens effective area.

However, the scanning lens using the conventional special toric surface,as described in (1), is a scanning lens in which the opposite surfacesare both anamorphic surfaces, and hence there are problems to be solvedin that:

-   -   beam spot diameter thickening occurs due to decentering; and    -   when the special toric surface is used for a surface having a        large angle of inclination, machining accuracy deteriorates,        thereby causing a defective image having thickened beam spot        diameter due to a form error such as swells, or vertical lines.

In the scanning lens using the conventional special toric surface, asdescribed in (1) and (2), the lens is thick, and even if the lens is aplastic molded article, the forming time and the cost of parts increase.Further, shading and ghost light have not been taken into consideration.

Further, in the scanning lens described in (1) and (2), a surface inwhich the radius of curvature in vertical scanning becomes asymmetricwith respect to the horizontal scanning direction is adopted, but arotationally symmetric aspheric surface is not used.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve at least the problemsin the conventional technology.

An optical scanner according to an aspect of the present inventionincludes a light source; an optical coupler that couples beams from thelight source; an optical line image unit that forms a line image of thebeams from the optical coupler, wherein the line image is longer in ahorizontal scanning direction than in a vertical scanning direction; adeflector that deflection-scans the beams from the optical line imageunit; and an optical scanning unit that includes scanning lenses thatguide the beams from the deflector to a surface to be scanned, wherein asurface close to the deflector, of the scanning lens closest to adeflection reflecting surface, has a negative power in the verticalscanning direction, and is a special toric surface in which a radius ofcurvature in a vertical scanning changes from an optical axis of thelens surface toward a periphery of the horizontal scanning direction,and wherein an F number of the beams toward the surface to be scanned ofthe scanning lens in the vertical scanning direction is larger in aperipheral part than in a central part in an effective scanning width.

An optical scanner according to another aspect of the present inventionincludes a light source; an optical coupler that couples beams from thelight source; an optical line image unit that forms a line image of thebeams from the optical coupler, wherein the line image is longer in ahorizontal scanning direction than in a vertical scanning direction; adeflector that deflection-scans the beams from the optical line imageunit; and an optical scanning unit that includes a plurality of scanninglenses that guide the beams from the deflector to a surface to bescanned. In this structure, incident beams in a most peripheral part inan effective write width are inclined in a same direction in adeflection surface of revolution with respect to a normal on a surfaceof a scanning lens close to the deflector, a scanning lens closest tothe deflector has a meniscus shape with a concave facing the deflectorside in the horizontal scanning direction, a surface close to thedeflector, of the scanning lens closest to the deflector, has a negativepower in the vertical scanning direction, and is a special toric surfacein which a radius of curvature in a vertical scanning changes from anoptical axis of the lens surface toward a periphery of the horizontalscanning direction, and the surface close to the deflector of a scanninglens farthest from the deflector has a positive power in the verticalscanning direction.

An optical scanner according to still another aspect of the presentinvention includes a light source; an optical coupler that couples beamsfrom the light source; an optical line image unit that forms a lineimage of the beams from the optical coupler, wherein the line image islonger in a horizontal scanning direction than in a vertical scanningdirection; a deflector that deflection-scans the beams from the opticalline image unit; and an optical scanning unit that includes a pluralityof scanning lenses that guide the beams from the deflector to a surfaceto be scanned. In this structure, a scanning lens closest to thedeflector has a meniscus shape with a concave facing the deflector sidein the horizontal scanning direction, a surface close to the deflector,of the scanning lens closest to the deflector, has a negative power inthe vertical scanning direction, and is a special toric surface in whicha radius of curvature in a vertical scanning decreases from an opticalaxis of the lens surface toward a periphery of the horizontal scanningdirection and increases bordering on an extreme value, and the surfaceclose to the deflector, of a scanning lens farthest from the deflector,has a convex shape toward the deflector side in the horizontal scanningdirection, and the scanning lens farthest from the deflector has anegative power in the horizontal scanning direction.

An optical scanner according to still another aspect of the presentinvention includes a light source; an optical coupler that couples beamsfrom the light source; an optical line image unit that forms a lineimage of the beams from the optical coupler, wherein the line image islonger in a horizontal scanning direction than in a vertical scanningdirection; a deflector that deflection-scans the beams from the opticalline image unit; and an optical scanning unit that includes a pluralityof scanning lenses that guide the beams from the deflector to a surfaceto be scanned. In this structure, at least one scanning lens has arotationally symmetric aspheric surface and a special toric surface inwhich a radius of curvature in a vertical scanning changes from anoptical axis of the lens surface toward a periphery of the horizontalscanning direction, and a surface having an area with a largest angle ofinclination in an effective range with respect to a surfaceperpendicular to the optical axis, among the surfaces of all scanninglenses, is the rotationally symmetric aspheric surface.

An image forming apparatus according to still another aspect of thepresent invention forms an image on transfer paper by executingrespective charging, exposure, development, and transfer processes. Theimage forming apparatus includes an exposing unit that executes theexposure process. The exposing unit includes a light source; an opticalcoupler that couples beams from the light source; an optical line imageunit that forms a line image of the beams from the optical coupler,wherein the line image is longer in a horizontal scanning direction thanin a vertical scanning direction; a deflector that deflection-scans thebeams from the optical line image unit; and an optical scanning unitthat includes scanning lenses that guide the beams from the deflector toa surface to be scanned. In this structure, a surface close to thedeflector, of the scanning lens closest to a deflection reflectingsurface, has a negative power in the vertical scanning direction, and isa special toric surface in which a radius of curvature in a verticalscanning changes from an optical axis of the lens surface toward aperiphery of the horizontal scanning direction, and wherein an F numberof the beams toward the surface to be scanned of the scanning lens inthe vertical scanning direction is larger in a peripheral part than in acentral part in an effective scanning width.

An image forming apparatus according to still another aspect of thepresent invention forms an image on transfer paper by executingrespective charging, exposure, development, and transfer processes. Theimage forming apparatus includes an exposing unit that executes theexposure process. The exposing unit includes a light source; an opticalcoupler that couples beams from the light source; an optical line imageunit that forms a line image of the beams from the optical coupler,wherein the line image is longer in a horizontal scanning direction thanin a vertical scanning direction; a deflector that deflection-scans thebeams from the optical line image unit; and an optical scanning unitthat includes a plurality of scanning lenses that guide the beams fromthe deflector to a surface to be scanned. In this structure, theincident beams in a most peripheral part in an effective write width areinclined in a same direction in a deflection surface of revolution withrespect to a normal on a surface of a scanning lens close to thedeflector, a scanning lens closest to the deflector has a meniscus shapewith a concave facing the deflector side in the horizontal scanningdirection, a surface close to the deflector, of the scanning lensclosest to the deflector, has a negative power in the vertical scanningdirection, and is a special toric surface in which a radius of curvaturein a vertical scanning changes from an optical axis of the lens surfacetoward a periphery of the horizontal scanning direction, and the surfaceto the deflector of a scanning lens farthest from the deflector has apositive power in the vertical scanning direction.

An image forming apparatus according to still another aspect of thepresent invention forms an image on transfer paper by executingrespective charging, exposure, development, and transfer processes. Theimage forming apparatus includes an exposing unit that executes theexposure process. The exposing unit includes a light source; an opticalcoupler that couples beams from the light source; an optical line imageunit that forms a line image of the beams from the optical coupler,wherein the line image is longer in a horizontal scanning direction thanin a vertical scanning direction; a deflector that deflection-scans thebeams from the optical line image unit; and an optical scanning unitthat includes a plurality of scanning lenses that guide the beams fromthe deflector to a surface to be scanned. In this structure, a scanninglens closest to the deflector has a meniscus shape with a concave facingthe deflector side in the horizontal scanning direction, a surface closeto the deflector, of the scanning lens closest to the deflector, has anegative power in the vertical scanning direction, and is a specialtoric surface in which a radius of curvature in a vertical scanningdecreases from an optical axis of the lens surface toward a periphery ofthe horizontal scanning direction and increases bordering on an extremevalue, the surface close to the deflector, of a scanning lens farthestfrom the deflector, has a convex shape toward the deflector side in thehorizontal scanning direction, and the scanning lens farthest from thedeflector has a negative power in the horizontal scanning direction.

An image forming apparatus according to still another aspect of thepresent invention forms an image on transfer paper by executingrespective charging, exposure, development, and transfer processes. Theimage forming apparatus includes an exposing unit that executes theexposure process. The exposing unit includes a light source; an opticalcoupler that couples beams from the light source; an optical line imageunit that forms a line image of the beams from the optical coupler,wherein the line image is longer in a horizontal scanning direction thanin a vertical scanning direction; a deflector that deflection-scans thebeams from the optical line image unit; and an optical scanning unitthat includes a plurality of scanning lenses that guide the beams fromthe deflector to a surface to be scanned. In this structure, at leastone scanning lens has a rotationally symmetric aspheric surface and aspecial toric surface in which a radius of curvature in a verticalscanning changes from an optical axis of the lens surface toward aperiphery of the horizontal scanning direction, and a surface having anarea with a largest angle of inclination in an effective range withrespect to a surface perpendicular to the optical axis, among thesurfaces of all scanning lenses, is the rotationally symmetric asphericsurface.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical arrangement in an optical scanneraccording to a first embodiment, as seen from the horizontal scanningdirection;

FIGS. 2A and 2B illustrate changes in the vertical scanning section ofbeams, FIG. 2A illustrating beams in the central part, and FIG. 2Billustrating beams in the most peripheral part;

FIG. 3 is an enlarged plan view of a scanning lens that is placedfarthest from a deflector in the optical scanner;

FIGS. 4A and 4B are aberration diagrams in a first example, FIG. 4Aillustrating a curvature of field, and FIG. 4B illustrating velocityuniformity;

FIG. 5 is a graph of changes in the radius of curvature with respect tothe lens height in the horizontal scanning direction, on the surface onthe deflector side of the scanning lens closest to the deflector in thefirst example;

FIG. 6 is a graph of changes in the radius of curvature with respect tothe lens height in the horizontal scanning direction, on the surface onthe deflector side of the scanning lens farthest from the deflector inthe first example;

FIG. 7 is a graph of changes in the radius of curvature with respect tothe lens height in the horizontal scanning direction, on a secondsurface of the scanning lens farthest from the deflector in the firstexample;

FIGS. 8A and 8B are graphs of beam spot diameter with respect todefocusing in the first example, FIGS. 8A and 8B being graphs showingbeam spot diameter in the horizontal scanning direction and in thevertical scanning section, respectively;

FIGS. 9A and 9B illustrate an optical arrangement for explaining awavefront aberration reduction effect in the first example, FIG. 9Aillustrating the optical system arrangement in the horizontal scanningdirection, and FIG. 9B illustrating the optical system arrangement inthe vertical scanning direction;

FIGS. 10A and 10B illustrate optical arrangement for explaining a ghostreduction effect of the optical scanner in the first example, FIGS. 10Aand 10B illustrating the optical arrangement in the horizontal andvertical scanning directions, respectively;

FIGS. 11A and 11B are aberration diagrams in a second example, FIG. 11Aillustrating a curvature of field, and FIG. 11B illustrating velocityuniformity;

FIG. 12 is a graph of changes in the radius of curvature with respect tothe lens height in the horizontal scanning direction, on the surface onthe deflector side of the scanning lens closest to the deflector in thesecond example;

FIG. 13 is a graph of changes in the radius of curvature with respect tothe lens height in the horizontal scanning direction, on the surface onthe deflector side of the scanning lens farthest to the deflector in thesecond example;

FIG. 14 is a graph of changes in the radius of curvature with respect tothe lens height in the horizontal scanning direction, on a secondsurface of the scanning lens farthest from the deflector in the secondexample;

FIGS. 15A and 15B are graphs of beam spot diameter with respect todefocusing in the second example, FIGS. 15A and 15B being graphs showingbeam spot diameter in the horizontal scanning direction and in thevertical scanning section, respectively; and

FIG. 16 is a schematic front elevation of an example of an image formingapparatus, to which the optical scanner according to the presentinvention can be applied.

DETAILED DESCRIPTION

Exemplary embodiments of an optical scanner and an image formingapparatus according to the present invention will be described belowwith reference to accompanying diagrams.

FIG. 1 illustrates an optical scanner according to a first embodiment. Alight source 1 in the optical scanner includes a semiconductor laser,and for example, may be a multi-beam light source in which a pluralityof light emitting sources are arranged at an equal interval. The beamsemitted from the light source 1 are divergent beams, and are coupledinto an optical system thereafter by a coupling lens 2 that constitutesan optical coupler of the present invention. The beams coupled by thecoupling lens 2 may be weak divergent beams or weak convergent beams, orparallel beams. The beams are subjected to beam forming by an aperture3, and the beams converge only in the vertical scanning direction by theaction of a cylindrical lens 4. Therefore, a line image of the beams,longer in the horizontal scanning direction than in the verticalscanning direction, is formed at a position close to a deflectionreflecting surface of a polygon mirror 5, being a deflector. Thecylindrical lens 4 and the polygon mirror 5, used for forming the lineimage of the beams, constitute an optical line image unit of the presentinvention.

The polygon mirror 5 is rotated at a high constant velocity by a polygonmotor, to deflection-scan the direction of the beams at an isometricvelocity by each deflection reflecting surface. A scanning opticalsystem is arranged on the course of the beams deflected at the isometricvelocity. The scanning optical system constitutes an optical scanningunit of the present invention. Thus, the scanning optical systemincludes two scanning lenses 7 and 8. The scanning lenses 7 and 8 have afunction of guiding the beams from the polygon mirror 5 to a surface 10that is to be scanned, being the surface of the photoconductor, andimaging the beams on the surface 10 as a beam spot. Further, thescanning lenses 7 and 8 perform the well-known fθ function of scanning,at a constant velocity, the beams deflected by the polygon mirror 5 atthe isometric velocity on the linear surface 10.

Here, the “horizontal scanning direction” corresponds to a plane formedby the beams to be deflection-scanned by the polygon mirror 5, and the“vertical scanning direction” is a direction orthogonal to thehorizontal scanning direction. The optical scanner also includes asoundproof glass 6, and a dustproof glass 9. The polygon mirror 5 andthe polygon motor are covered with a soundproof cover so that noisegenerated by the high-speed rotation does not leak to the outside, andthe beams enter or go out through the soundproof glass 6 provided inthis cover. The optical scanner is incorporated in one housing as oneunit, is sealed so that dust and dirt do not enter the housing, and thedeflected beams are emitted through the dustproof glass 9.

The optical scanner according to the first embodiment has a feature inthe configuration of the scanning optical system. Therefore, theconfiguration of the scanning optical system will be explainedspecifically. It is assumed herein that the surface of the lens in thescanning optical system, on the side close to the polygon mirror 5, thatis, the deflector is a first surface, and the surface on the side awayfrom the deflector is a second surface.

The first surface of a scanning lens 7 closest to the deflectionreflecting surface has a negative power in the vertical scanningdirection, and is a special toric surface in which the radius ofcurvature in vertical scanning changes from the optical axis of the lenssurface toward the periphery of the horizontal scanning direction. Thefirst surface of the scanning lens 7 has the negative power, to reducethe absolute value of the lateral magnification in the vertical scanningdirection between the deflection reflecting surface and the surface 10.By reducing the absolute value of the lateral magnification in thevertical scanning direction, variations in the beam waist position inthe vertical scanning direction due to an installation error or a shapeerror of the optical parts reduce.

In the optical scanner according to the first embodiment, a secondscanning lens 8, that is, the scanning lens away from the deflector isuniformly thin (see FIG. 3), and it is normally difficult to correct thewavefront aberration in the scanning optical system that includes thesecond scanning lens 8 of such a shape. When it is tried to correct thewavefront aberration, the vertical scanning curvature of the firstsurface of the first scanning lens 7 increases from the center towardthe circumference. At this time, however, if the F number in thevertical scanning direction on the surface 10 is made constant by theimage height, the second scanning lens 8 has a shape largely curved in aconvex shape toward the deflector side within the deflection surface ofrevolution. This ensures the characteristic at the medium value indesign, but the beam spot diameter thickening due to decenteringincreases, and the optical scanning performance deteriorates as a whole.

The F number is defined as the ratio of the focal length f of the lensto the effective diameter D of the lens and the value of the F number isobtained as F=f/D.

Therefore, in this embodiment, the scanning optical system isconstructed such that the F number in the vertical scanning direction ofthe beams directed to the surface 10 increases in the most peripheralpart in the effective scanning width than in the central part thereof,at such a level that a beam spot diameter deviation in the verticalscanning direction due to an image height or a beam pitch deviation dueto the image height, when the light source is a multi-beam light source,does not cause a problem. As a result, the optical scanner, in which thewavefront aberration at the medium value can be reduced, and which has alarge tolerance for decentering, can be provided.

The F number in the vertical scanning direction will be explainedfurther. To reduce the absolute value of the lateral magnification inthe vertical scanning direction and correct the wavefront aberration, itis necessary to make the power in the vertical scanning direction of thefirst surface of the scanning lens 7 on the deflector side negative, andto increase the power, that is, increase the curvature from the centralpart toward the peripheral part in the horizontal scanning direction, toreduce the negative power. In FIG. 1, the beams indicated by a one-dotchain line denote the beams in the central part, and the beams indicatedby a dotted line denote the beams in the most peripheral part. FIGS. 2Aand 2B illustrate changes in the vertical scanning section of beams fromthe deflection reflecting surface to the surface 10, FIG. 2Aillustrating the beams in the central part, and FIG. 2B illustrating thebeams in the most peripheral part. The power in the vertical scanningdirection of the first surface of the scanning lens 7 is larger in theperipheral part than in the central part. Therefore, if it is assumedthat the convergent angle in the central part of the beams converging ina spot form on the surface 10 is θ1, and the convergent angle in themost peripheral part is θ2, then θ1 is larger than θ2. Therefore, the Fnumber in the vertical scanning direction on the surface 10 is larger inthe most peripheral part than in the central part. In other words, thewavefront aberration can be favorably corrected by setting the F numberin the vertical scanning direction on the surface 10 to the relationdescribed above.

In the first embodiment, it is desired that the surface away from thedeflector (second surface) of the scanning lens 7 closest to thedeflection reflecting surface be a rotationally symmetric asphericsurface centering on the optical axis. The rotationally symmetricaspheric surface has an advantage in that it can be formed relativelyeasily.

All surfaces of the scanning lens may be the special toric surface inwhich the radius of curvature in vertical scanning changes from theoptical axis of the lens surface toward the periphery of the horizontalscanning direction.

Generally, the surface away from the deflection reflecting surface, ofthe scanning lens closest to the deflector, has a large angle ofinclination (the angle of inclination of the lens surface with respectto the surface perpendicular to the optical axis). Consequently, theproblem in machining increases. Thus, in view of the opticalcharacteristic and machining, the rotationally symmetric asphericsurface is effective. If one surface is the rotationally symmetricaspheric surface, there is an advantage in that the beam spot diameterthickening due to the relative deviation between surfaces reduces.

The optical scanner according to a second embodiment will be explainednext. In the second embodiment, the arrangement of the optical elementsfrom the light source 1 to the surface 10 is substantially the same asthat in the first embodiment. Therefore, the optical scanner of thesecond embodiment will be explained with reference to FIG. 1. Also inthis embodiment, there is a feature in the configuration of the scanningoptical system, and hence the configuration of the scanning opticalsystem will be mainly explained. The scanning optical system includes aplurality of scanning lenses 7 and 8. The incident beams in the mostperipheral part in the effective write width is inclined in the samedirection in the deflection surface of revolution, with respect to thenormal on the first surface of the scanning lens 7. The scanning lens 7that is closest to the deflector has a meniscus shape with the concavefacing the deflector side in the horizontal scanning direction. Thefirst surface of the scanning lens 7 has a negative power in thevertical scanning direction, and is the special toric surface in whichthe vertical scanning curvature changes from the optical axis of thelens surface toward the periphery of the horizontal scanning direction.The first surface of the scanning lens 8 that is farthest from thedeflector has a positive power in the vertical scanning direction.

Since the first scanning lens 7 has the meniscus shape, with the concavefacing the polygon mirror 5 in the horizontal scanning direction, thescanning lens can be made thin, and the angle between the beams incidenton the surface of the scanning lens 7 and the normal on the lens surfacecan be decreased. Consequently, the wavefront aberration can becorrected. Making the power in the vertical scanning direction of thefirst surface of the first scanning lens 7 negative, reduces theabsolute value of the lateral magnification in the vertical scanningdirection of the scanning optical system, and enlarges the tolerance foran installation error or a part error of the optical elements. Moreover,using the special toric surface in which the radius of curvature invertical scanning changes from the optical axis toward the periphery ofthe horizontal scanning direction, reduces both the wavefront aberrationand a difference in the F number in the vertical scanning direction onthe surface to be scanned. Consequently, the deviation of the verticalscanning beam spot diameter due to the image height reduces. Further, abeam pitch deviation between image heights at the time of usingmulti-beams reduces.

However, if the power in the vertical scanning direction on the firstsurface of the first scanning lens 7 closest to the deflector isnegative, even if the special toric surface is used, the wavefrontaberration remains. Therefore, as shown in FIGS. 9A and 9B, the scanningoptical system is constructed such that the beams in the most peripheralpart in the scanning optical system are inclined in the same directionwith respect to the normal on the first surface of the first scanninglens 7 and on the first surface of the second scanning lens 8.Consequently, the tolerance for decentering of the first scanning lens 7and the second scanning lens 8 increases.

Japanese Patent Application Laid-Open No. 2001-21824 discloses a methodof decreasing the angle between the incident beams and the scanninglens. However, in this method, it is necessary to largely curve thescanning lens within the deflection surface of revolution. Consequently,the workability of the scanning lens decreases, and the tolerance fordecentering considerably decreases. Therefore, in the second embodiment,the power in the vertical scanning direction on the first surface of thesecond scanning lens 8 is made positive, so that the wavefrontaberration occurring on the first surface of the first scanning lens 7is compensated by the first surface of the second scanning lens 8.

If the first scanning lens 7 has a meniscus shape with the concavedirected toward the polygon mirror in the horizontal scanning direction,as shown by two-dot chain line in FIG. 10, then there is the possibilitythat the ghost light reflected from the first surface of the secondscanning lens 8 is reflected again onto the first or the second surfaceof the first scanning lens 7, and as shown by arrow of the dotted line,the reflected light reaches the surface 10, thereby causing the ghostlight. Further, there is the possibility that the ghost lights on thefirst surface and the second surface of the first scanning lens 7combine and increase the intensity of the ghost light on the surface 10.However, if the power in the vertical scanning direction on the firstsurface of the second scanning lens 8 is made positive, in other words,if the first surface of the second scanning lens 8 is a convex planetoward the deflector (as shown by a solid line in FIG. 10), the ghostlight reflected by the first surface of the second scanning lens 8diverges to such a level that causes no problem. The beams reflected bythe first scanning lens 7 and returning to the polygon mirror change thedirection largely. Hence, generally, there is no problem.

The optical scanner according to a third embodiment will be explainednext. In this embodiment, the appearance is the same as those shown inFIGS. 1, 9A and 9B. The scanning optical system includes a plurality ofscanning lenses 7 and 8. The first scanning lens 7 closest to thedeflector has a meniscus shape, which is concave toward the deflectorside in the horizontal scanning direction. The first surface of thescanning lens 7 that is closest to the deflector has a negative power inthe vertical scanning direction, and is the special toric surface inwhich the radius of curvature in the vertical scanning decreases fromthe optical axis of the lens surface toward the periphery of thehorizontal scanning direction, and increases bordering on an extremevalue. The first surface of the second scanning lens 8 farthest from thedeflector has a convex shape toward the deflector side in the horizontalscanning direction, and the second scanning lens 8 farthest from thedeflector has a negative power in the horizontal scanning direction.

As in the second embodiment, the first surface of the first scanninglens 7 is the special toric surface, and has a negative power in orderto reduce the absolute value of the lateral magnification in thevertical scanning direction between the deflection reflecting surfaceand the surface 10. Reducing the absolute value of the lateralmagnification in the vertical scanning direction reduces the variationsin the beam waist position in the vertical scanning direction due to aninstallation error or a shape error of the optical parts. The scanninglens in the third embodiment, more specifically, the second scanninglens 8, is uniformly thin, and it is normally difficult to correct thewavefront aberration. To correct the wavefront aberration, the radius ofcurvature in vertical scanning on the first surface of the firstscanning lens 7 is increased from the center toward the peripheral part.Making the second scanning lens 8 convex toward the deflector side inthe horizontal scanning direction, makes the F number in the verticalscanning direction on the surface 10 constant due to the image height.

If the first scanning lens 7 has the meniscus shape, with the concavefacing the polygon mirror in the horizontal scanning direction asdescribed above, the ghost light reflected on the first surface of thesecond scanning lens 8 is again reflected on the first or the secondsurface of the first scanning lens 7 to reach the surface 10 again,thereby causing the ghost light. Further, there is the possibility thatthe ghost lights on the first and the second surfaces of the firstscanning lens 7 combine and increase the intensity of the ghost light onthe surface 10. However, in the third embodiment, the first surface ofthe second scanning lens 8 is made convex toward the deflector side inthe horizontal scanning direction. Therefore, the ghost light returningto the first scanning lens 7 reduces, and hence, the image quality doesnot degrade. Similar to the above embodiments, if the power of the firstsurface of the second scanning lens 8 in the vertical scanning directionis made positive, the effect further increases.

If the second scanning lens 8 has a concave shape toward the deflector,as shown by chain line in FIG. 10A, the ghost light is likely to occur.However, if the image forming apparatus has a configuration as in thethird embodiment, the ghost light occurs rarely.

In order to make the second scanning lens 8 thin, it is desired that thepower on the optical axis in the horizontal scanning direction isnegative, and the thickness thereof increases from the center toward theperipheral part, and decreases toward the peripheral part, bordering onthe extreme value. By making the power of the second scanning lens 8 inthe horizontal scanning direction negative, variations in the curvatureof field in the horizontal scanning direction due to temperature reduce,and a stable beam spot diameter can be obtained.

The optical scanner according to a fourth embodiment will be explainedbelow. In this embodiment, the appearance is the same as those shown inFIGS. 1, 9A, and 9B, and has a feature in the configuration of thescanning optical system. Therefore, the configuration of the scanningoptical system will be specifically explained.

In the optical scanner according to the fourth embodiment, the scanningoptical system includes a plurality of scanning lenses 7 and 8, andsatisfies the conditions given below:

-   -   at least one scanning lens has a rotationally symmetric aspheric        surface, and a special toric surface in which the radius of        curvature in vertical scanning changes from the optical axis of        the lens surface toward the periphery of the horizontal scanning        direction; and    -   the surface having an area with the largest angle of inclination        in the effective range with respect to the surface perpendicular        to the optical axis, among the surfaces of all scanning lenses,        is the rotationally symmetric aspheric surface.

More specifically, in the first scanning lens 7 closest to thedeflection reflecting surface in the scanning optical system, the firstsurface is the special toric surface in which the radius of curvature invertical scanning changes from the optical axis of the lens surfacetoward the periphery of the horizontal scanning direction, and thesecond surface is the rotationally symmetric aspheric surface. Therotationally symmetric aspheric surface here is symmetric about theoptical axis of the lens as the axis of symmetry.

Thus, using the special toric surface for the first surface of the firstscanning lens 7, reduces the wavefront aberration in the design, andreduces a difference in the F number in the vertical scanning directionon the surface 10 due to the image height, reduces a deviation in thebeam spot diameter in the vertical scanning due to the image height, andalso reduces a beam pitch deviation between image heights if multi-beamsare used. Among all the scanning lenses, the surface having the largestangle of inclination with respect to the optical axis is the secondsurface of the first scanning lens 7. Using the rotationally symmetricaspheric surface for this surface, beam spot diameter thickening due torelative deviation between respective surfaces reduces, therebyrealizing stabilization of the beam spot diameter. The scanning lens ismachined by cutting or forming using plastic or the like, and theforming die is machined by cutting. Therefore, if the surface of thelens is the rotationally symmetric aspheric surface, the working timereduces, and working accuracy improves, thereby improving the quality.

The optical scanner according to a fifth embodiment will be explainednext.

In the optical scanner according to the fifth embodiment, in addition tothe above configuration, the surfaces of all scanning lenses 7 and 8 inthe scanning optical system are formed of the rotationally symmetricaspheric surface or the special toric surface.

The rotationally symmetric aspheric surface is a surface having largetolerance for decentering and excellent workability, and the specialtoric surface is a surface excellent in aberration correction in thedesign. There are a cylindrical lens surface, a general toric surface,and the like, but none of those surfaces have large tolerance fordecentering, and do not have the capacity for aberration correction onthe design as large as the special toric surface. Therefore, by formingall surfaces of the scanning lenses 7 and 8 in the scanning opticalsystem into the special toric surface and the rotationally symmetricaspheric surface, an optical scanner that is excellent in workabilityand that has a large tolerance for decentering and excellent opticalcharacteristics, can be provided.

Both surfaces of the second scanning lens 8 here are the special toricsurface, to reduce the wavefront aberration on the design, and to reducea difference in the F number in the vertical scanning direction on thesurface 10 due to the image height.

The optical scanner according to a sixth embodiment will be explainednext.

In the optical scanner according to the sixth embodiment, in addition tothe configuration above, the surface having the largest effective widthin the horizontal scanning direction, among the surfaces of the scanninglenses 7 and 8 in the scanning optical system, is the special toricsurface.

The surface having the largest effective width in the horizontalscanning direction, of the surfaces of the scanning lenses 7 and 8, isthe second surface of the second scanning lens 8. Using the specialtoric surface for this surface, the curvature of field in the verticalscanning direction can be finely corrected.

The optical scanner according to a seventh embodiment will be explainednext.

In the optical scanner according to the seventh embodiment, in additionto the configuration above, all scanning lenses 7 and 8 in the scanningoptical system have an angle of inclination on the lens surface of 30degrees or less in the effective range with respect to the surfaceperpendicular to the optical axis. The scanning lenses 7 and 8 have thespecial toric surface in which the radius of curvature in verticalscanning changes according to the lens height in the horizontal scanningdirection. In the special toric surface, a variation in the verticalscanning curvature per 1 millimeter (mm) of the lens height in thehorizontal scanning direction is smaller than 1.5×10⁻⁴ (1/mm).

The working method of the scanning lens here is largely divided intocutting and forming, but the forming die is manufactured by cutting. Theangle of inclination on the lens surface in the effective range largelyrelates to the accuracy of cutting, and it is necessary to make theangle of inclination of all scanning lenses 7 and 8 equal to or lessthan 30 degrees, to reduce swells. In the present invention, a specialsurface in which the vertical scanning curvature changes according tothe lens height in the horizontal scanning direction is used, and it isnecessary that the change in the lens height in the horizontal scanningdirection be as small as possible. Therefore, to make the wavefrontaberration favorable, and to obtain a stable beam spot diameter, it isnecessary that the variation in the vertical scanning curvature per 1 mmof the lens height in the horizontal scanning direction be smaller than1.5×10⁻⁴ (1/mm) in the special toric surface. If the variation in thevertical scanning curvature per 1 mm of the lens height in thehorizontal scanning direction is large, deterioration in the beam spotdiameter due to the installation error of the lens increases. Thepresent invention is effective in this regard.

The optical scanner according to an eighth embodiment will be explainednext.

In the optical scanner according to the eighth embodiment, in additionto the configuration above, the scanning optical system includes twoscanning lenses 7 and 8, and satisfies the conditions given below:

-   -   the beams in the most peripheral part in the effective write        width are inclined in the same direction in the deflection        reflecting surface, with respect to the normal on the first        surfaces of the two scanning lenses 7 and 8;    -   the first scanning lens 7 closest to the polygon mirror 5 has a        meniscus shape, with the concave facing the deflector side in        the horizontal scanning direction;    -   the first surface of the first scanning lens 7 closest to the        polygon mirror 5 has a negative power in the vertical scanning        direction, and is formed of the special toric surface in which        the vertical scanning curvature changes from the optical axis of        the lens surface toward the periphery of the horizontal scanning        direction; and    -   the first surface of the second scanning lens 8 farthest from        the polygon mirror 5 has a positive power in the vertical        scanning direction.

The first scanning lens 7 here has the meniscus shape, with the concavefacing the polygon mirror 5 in the horizontal scanning direction.Therefore, the first scanning lens 7 can be made thin, and the anglebetween the beams incident on the surface of the first scanning lens 7and the normal on the lens surface can be reduced, thereby enablingcorrection of the wavefront aberration. Further, by making the power inthe vertical scanning direction on the first surface of the firstscanning lens 7 negative, the absolute value of the lateralmagnification in the vertical scanning direction of the scanning opticalsystem can be reduced, thereby enlarging the tolerance for aninstallation error and parts error of the optical elements. Using thespecial toric surface in which the radius of curvature in the verticalscanning decreases from the optical axis toward the peripheral part ofthe horizontal scanning direction, and increases bordering on theextreme value, the wavefront aberration and a difference in the F numberin the vertical scanning direction on the surface can be reduced, adeviation in the beam spot diameter in the vertical scanning due to theimage height can be reduced, and a beam pitch deviation between imageheights at the time of using the multi-beams can be reduced.

However, if the power in the vertical scanning direction of the firstsurface of the first scanning lens 7 is negative, even when the specialtoric surface is used, the wavefront aberration remains. Therefore, asshown in FIGS. 9A and 9B, the scanning optical system is constructedsuch that the beams in the most peripheral part in the scanning opticalsystem are inclined in the same direction with respect to the normal onthe first surface of the first scanning lens 7 and on the first surfaceof the second scanning lens 8. Consequently, the tolerance fordecentering of the first scanning lens 7 and the second scanning lens 8increases. As described in Japanese Patent Application Laid-Open No.2001-21824, there is a method of decreasing the angle between theincident beams and the scanning lens. However, in this case, since it isnecessary to largely curve the scanning lens in the deflection surfaceof revolution, not only does the workability of the scanning lensdecrease, but also the tolerance for decentering considerably decreases.In the above configuration, therefore, the power in the verticalscanning direction on the first surface of the second scanning lens 8 ismade positive, so that the wavefront aberration occurring on the firstsurface of the first scanning lens 7 is compensated by the first surfaceof the second scanning lens 8.

As shown by one-dot chain line in FIG. 10A, if the second scanning lens8 has a meniscus shape, with the concave directed toward the polygonmirror in the horizontal scanning direction, there is the possibilitythat the ghost light reflected on the first surface of the secondscanning lens 8 is again reflected on the first or the second surface ofthe first scanning lens 7, as shown by arrow of dotted line, thereflected light reaches the surface 10, thereby causing the ghost light.Further, there is the possibility that the ghost lights on the firstsurface, and those on the second surface of the first scanning lens 7are combined, to increase the intensity of the ghost light on thesurface 10. However, as shown by solid line in FIGS. 10A and 10B, if thepower in the vertical scanning direction on the first surface of thesecond scanning lens 8 is made positive, in other words, if the firstsurface of the second scanning lens 8 is a convex plane toward thedeflector, the ghost light reflected by the first surface of the secondscanning lens 8 diverges to such a level that causes no problem. Thebeams reflected by the first scanning lens 7 and returning to thepolygon mirror change the direction largely, and hence, generally thereis no problem.

The optical scanner explained in the first to the eighth embodiments canbe applied as an exposure unit or a write unit in an image formingapparatus such as a printer or a copier. FIG. 16 is a schematic frontelevation of an example of an image forming apparatus to which theoptical scanner according to the present invention is applied as anexposure unit. The exposure unit constitutes an exposing unit of thepresent invention. In FIG. 16, a charging unit 112, an exposure unit117, a developing unit 113, a transfer unit 114, and a cleaning unit 115are arranged in this order in the direction of rotation of aphotosensitive drum 111 in an image forming apparatus 100, to execute anelectrophotographic process of charging, exposure, development,transfer, and cleaning. The exposure unit 117 includes the opticalscanner, and emits laser beams LB for deflection scanning toward thesurface of the photosensitive drum 111, so that the laser beam spotscans the surface of the photosensitive drum 111.

The surface of the photosensitive drum 111 is uniformly chargedbeforehand by the charging unit 112, and by scanning the surface by thebeam spot modulated according to an image signal, an electrostaticlatent image is formed on the surface of the photosensitive drum 111.The developing unit 113 supplies a toner to visualize the electrostaticlatent image as a toner image. The transfer unit 114 transfers the tonerimage onto transfer material (transfer paper, or various kinds of sheet)P supplied one by one from a paper feed cassette 118 by a paper feedroller 120, with the timing adjusted by a resist roller 119. The surfaceof the photosensitive drum 111 after the transfer is discharged andcleaned by the cleaning unit 115, and charged again. On the other hand,the transfer paper P onto which the toner image has been transferred isheated and fixed by a fixing unit 116, and discharged to a catch tray123, through a discharge path 121 and discharge rollers 122.

In the image forming apparatus having such a configuration, by includingthe optical scanner having the configuration described above as themeans for executing the exposure process, the wavefront aberration canbe made favorable, a stable beam spot diameter can be obtained,deterioration in the beam spot diameter due to decentering can bereduced, and the ghost light can be removed. As a result, the beam spothaving a small diameter can be realized, thereby enabling imageformation that has excellent granularity and gradient.

In FIG. 16, an image forming apparatus that forms a monochrome image isshown. However, an image forming apparatus that forms multi-color orfull-color images can be obtained by having a configuration such that aplurality of imaging units including the photosensitive drum 111, andthe various units around the photosensitive drum 111, that is, thecharging unit, the exposure unit, the developing unit, the transferunit, and the cleaning unit, are arranged in parallel in thetransportation direction of the transfer material (in a tandemconfiguration). When the configuration is such that developing units ofa plurality of colors and an intermediate transfer body are providedwith respect to one photosensitive drum, one drum and intermediatetransfer type color image forming apparatus can be obtained. Using theoptical scanner of the present invention in the color image formingapparatus, color image formation that has excellent granularity andgradient can be performed.

A first example of the optical scanner is explained next, as a specificexample.

The specification from the light source to the polygon mirror 5 as thedeflector is as follows:

-   -   Wavelength of light source 1: 655 nanometer (nm)    -   Focal length of coupling lens 2: 27 mm    -   Coupling action: collimating action    -   Polygon mirror 5:        -   Number of deflection reflecting surfaces: 5        -   Inscribed circle radius: 18 mm    -   Angle between incident angle of beams from light source side and        optical axis of scanning optical system: 58 degrees.

The lens data after deflection is as follows.

The first surface of the first scanning lens 7 and both surfaces of thesecond scanning lens 8 are expressed by the equations (1) and (2) asexplained below.

Horizontal Scanning Noncircular Equation:

The surface in the horizontal scanning surface is of noncircular shape.Assuming that the paraxial radius of curvature in the horizontalscanning surface on the optical axis is Rm, the distance in thehorizontal scanning direction from the optical axis is Y, the conicalconstant is K, and higher-order coefficients are A1, A2, A3, A4, A5, A6,. . . , the depth in the direction of the optical axis X is expressed bythe following polynomial (1).X=(Y^2/Rm)/[1+√{1−(1+K)(Y/Rm)^2}]+A1·Y+A2·Y^2+A3·Y^3+A4·Y^4+A5·Y^5+A6·Y^6+  (1)

Here, if a numerical value other than zero is substituted for odd-ordercoefficients A1, A3, A5, . . . , then the depth has an asymmetric shapein the horizontal scanning direction.

In the first example, only even-orders are used, giving a system that issymmetric in the horizontal scanning direction. Similarly, in a secondexample described later, only even-orders are used, giving a systemsymmetric in the horizontal scanning direction.

Vertical Scanning Curvature Equation:

An equation (2) in which the vertical scanning curvature changescorresponding to the horizontal scanning direction is shown below.Cs(Y)=1/Rs(0)+B1·Y+B2·Y^2+B3·Y^3+B4·Y^4+B5·Y^5+  (2)

Here, if a numerical value other than zero is substituted for odd-ordercoefficients B1, B3, B5, . . . , the radius of curvature in verticalscanning becomes asymmetric in the horizontal scanning direction.

The second surface of the first scanning lens 7 is a rotationallysymmetric aspheric surface, and expressed by the following equation (3).

Rotationally Symmetric Aspheric Surface:

Assuming that the paraxial radius of curvature on the optical axis is R,the distance in the horizontal scanning direction from the optical axisis Y, the conical constant is K, and higher-order coefficients are A1,A2, A3, A4, A5, A6, . . . , the depth in the direction of the opticalaxis X is expressed by the following polynomial (3).X=(Y^2/R)/[1+√{1−(1+K)(Y/Rm)^2}]+A1·Y+A2·Y^2+A3·Y^3+A4·Y^4+A5·Y^5+A6·Y^6+  (3)

Specific numerical data is shown below. In the numerical data below,[×10⁺¹] is expressed as [E+01], and [×10⁻⁷] is expressed as [E−07], andthe same thing applies hereafter.

Shape of First Surface of First Scanning Lens 7:

-   -   Rm=−279.9, Rs=−61.0    -   K=−2.900000E+01    -   A4=1.755765E−07    -   A6=−5.491789E−11    -   A8=1.087700E−14    -   A10=−3.183245E−19    -   A12=−2.635276E−24    -   B1=−2.066347E−06    -   B2=5.727737E−06    -   B3=3.152201E−08    -   B4=2.280241E−09    -   B5=−3.729852E−11    -   B6=−3.283274E−12    -   B7=1.765590E−14    -   B8=1.372995E−15    -   B9=−2.889722E−18    -   B10=−1.984531E−19        Shape of Second Surface of First Scanning Lens 7:    -   R=−83.6    -   K=−0.549157    -   A4=2.748446E−07    -   A6=−4.502346E−12    -   A8=−7.366455E−15    -   A10=1.803003E−18    -   A12=2.727900E−23        Shape of First Surface of Second Scanning Lens 8:    -   Rm=6950, Rs=110.9    -   K=0.000000E+00    -   A4=1.549648E−08    -   A6=1.292741E−14    -   A8=−8.811446E−18    -   A10=−9.182312E−22    -   B1=−9.593510E−07    -   B2=−2.135322E−07    -   B3=−8.079549E−12    -   B4=2.390609E−12    -   B5=2.881396E−14    -   B6=3.693775E−15    -   B7=−3.258754E−18    -   B8=1.814487E−20    -   B9=8.722085E−23    -   B10=−1.340807E−23        Shape of Second Surface of Second Scanning Lens 8:    -   Rm=766, Rs=−68.22    -   K=1.000000E+00    -   A4=−1.150396E−07    -   A6=1.096926E−11    -   A8=−6.542135E−16    -   A10=1.984381E−20    -   A12=−2.411512E−25    -   B2=3.644079E−07    -   B4=−4.847051E−13    -   B6=−1.666159E−16    -   B8=4.534859E−19    -   B10=−2.819319E−23

The refractive index of the scanning lens in the used wavelength is1.52724 in all lenses.

Specific numerical values in the optical arrangement are as follows:

Distance d1 from deflected surface to first surface of first scanninglens: 64 mm;

Thickness d2 at the center of first scanning lens 7: 22.6 mm;

Distance d3 from second surface of first scanning lens to first surfaceof second scanning lens: 75.9 mm;

Thickness d4 at the center of second scanning lens: 4.9 mm;

Distance d5 from second surface of second scanning lens to surface to bescanned: 158.7 mm.

The soundproof glass 6 and the dustproof glass 9 having a refractiveindex of 1.514 and a thickness of 1.9 mm are arranged as shown in FIGS.9A and 9B, and the soundproof glass 6 is inclined by 10 degrees withrespect to a direction parallel to the horizontal scanning direction inthe deflection surface of revolution.

The F numbers in the vertical scanning direction at the image height inthe most peripheral part and the central part of the scanning opticalsystem are given below:

-   -   Image height 150 mm: 41.5,    -   Image height 0 mm: 40.4,    -   Image height −150 mm: 41.0.

FIGS. 4A and 4B are aberration diagrams of the first example asdescribed above. FIG. 4A illustrates the curvature of field, whereX-axis indicates defocusing (mm), and Y-axis indicates image height(mm). The solid line indicates the curvature of field in the verticalscanning direction, and the dotted line indicates the curvature of fieldin the horizontal scanning direction. FIG. 4B illustrates the velocityuniformity, where X-axis indicates percentage, and Y-axis indicatesimage height (mm). The solid line indicates linearity, and the dottedline indicates fθ characteristic.

FIG. 5 is a graph of changes in the radius of curvature in the verticalscanning direction with respect to the lens height in the horizontalscanning direction, on the first surface (R1) of the first scanning lens(L1) 7.

FIG. 6 is a graph of changes in the radius of curvature in the verticalscanning direction with respect to the lens height in the horizontalscanning direction, on the first surface (R1) of the second scanninglens (L2) 8.

FIG. 7 is a graph of changes in the radius of curvature in the verticalscanning direction with respect to the lens height in the horizontalscanning direction, on the second surface (R2) of the second scanninglens (L2) 8.

FIGS. 8A and 8B are graphs of beam spot diameter with respect todefocusing, where FIG. 8A illustrates beam spot diameter in thehorizontal scanning direction, and FIG. 8B illustrates beam spotdiameter in the vertical scanning direction.

The maximum inclination angle of all lens surfaces is 28 degrees of thesecond surface of the first scanning lens 7.

On the first surface of the first scanning lens 7, and on the first andthe second surfaces of the second scanning lens 8, which are the specialtoric surfaces, the variations in the vertical scanning curvature per 1mm of the lens height on the respective surfaces are sequentially asfollows:

-   -   7.9E−05 (1/mm),    -   7.1E−05 (1/mm),    -   1.4E−04 (1/mm).

A second example of the optical scanner is explained next.

-   -   Wavelength of light source: 655 nm    -   Focal length of coupling lens: 27 mm    -   Coupling action: collimating action    -   Polygon mirror:        -   Number of deflection reflecting surfaces: 5        -   Inscribed circle radius: 18 mm    -   Angle between incident angle of beams from light source side and        optical axis of scanning optical system: 58 degrees.

The lens data after deflection is as follows.

The first surface of the first scanning lens 7 and both surfaces of thesecond scanning lens 8 are expressed by the following equations (1) and(2).

Horizontal Scanning Noncircular Equation:

The surface in the horizontal scanning surface is of noncircular shape.Assuming that the paraxial radius of curvature in the horizontalscanning surface on the optical axis is Rm, the distance in thehorizontal scanning direction from the optical axis is Y, the conicalconstant is K, and higher-order coefficients are A1, A2, A3, A4, A5, A6,. . . , the depth in the direction of the optical axis X is expressed bythe following polynomial (1).X=(Y^2/Rm)/[1+√{1−(1+K)(Y/Rm)^2}]+A1·Y+A2·Y^2+A3·Y^3+A4·Y^4+A5·Y^5+A6·Y^6+  (1)

Here, if a numerical value other than zero is substituted for odd-ordercoefficients A1, A3, A5, . . . , then the depth has an asymmetric shapein the horizontal scanning direction.

Vertical Scanning Curvature Equation:

An equation (2) in which the vertical scanning curvature changescorresponding to the horizontal scanning direction is shown below.Cs(Y)=1/Rs(0)+B1·Y+B2·Y^2+B3·Y^3+B4·Y^4+B5·Y^5+  (2)

Here, if a numerical value other than zero is substituted for odd-ordercoefficients B1, B3, B5, . . . , the radius of curvature in verticalscanning becomes asymmetric in the horizontal scanning direction.

The second surface of the first scanning lens 7 is the rotationallysymmetric aspheric surface, and expressed by the following equation (3).

Rotationally Symmetric Aspheric Surface:

When it is assumed that the paraxial radius of curvature on the opticalaxis is R, the distance in the horizontal scanning direction from theoptical axis is Y, the conical constant is K, and higher-ordercoefficients are A1, A2, A3, A4, A5, A6, . . . , the depth in thedirection of optical axis X is expressed by the following polynomial.X=(Y^2/R)/[1+√{1−(1+K)(Y/Rm)^2}]+A1·Y+A2·Y^2+A3·Y^3+A4·Y^4+A5·Y^5+A6·Y^6+  (3)

Specific numerical data is shown below. In the numerical data below,[×10⁺¹] is expressed as [E+01], and [×10⁻⁷] is expressed as [E−07], andthe same thing applies hereafter.

Shape of First Surface of First Scanning Lens 7:

-   -   Rm=−303.54, Rs=−61.0    -   K=−2.900000E+01    -   A4=2.28E−07    -   A6=−6.57E−11    -   A8=1.18E−14    -   A10=−2.10E−19    -   A12=8.00E−24    -   B1=−1.00E−06    -   B2=5.22E−06    -   B3=1.70E−08    -   B4=−5.06E−11    -   B5=−6.80E−12    -   B6=−9.46E−14    -   B7=−7.34E−16    -   B8=−2.10E−17    -   B9=−5.03E−19    -   B10=7.51E−21        Shape of Second Surface of First Scanning Lens 7:    -   R=−85.6    -   K=−0.549157    -   A4=2.83E−07    -   A6=6.04E−12    -   A8=−1.18E−14    -   A10=2.26E−18    -   A12=6.61E−23        Shape of First Surface of Second Scanning Lens 8:    -   Rm=6950, Rs=94.4    -   K=0.000000E+00    -   A4=1.13E−08    -   A6=9.27E−14    -   A8=−2.16E−19    -   A10=−9.18E−22    -   B1=−4.41E−07    -   B2=−6.96E−08    -   B3=−7.45E−11    -   B4=1.37E−11    -   B5=−6.44E−16    -   B6=−3.81E−15    -   B7=3.04E−18    -   B8=4.21E−19    -   B9=−2.33E−22    -   B10=−1.55E−23        Shape of Second Surface of Second Scanning Lens 8:    -   Rm=781.2, Rs=−76.09    -   K=0.000000E+00    -   A4=−1.14E−07    -   A6=9.25E−12    -   A8=−3.65E−16    -   A10=9.51E−22    -   A12=2.38E−25    -   B2=4.91E−07    -   B4=−1.64E−11    -   B6=7.96E−16    -   B8=1.30E−19    -   B10=1.47E−23

The refractive index of the scanning lens in the used wavelength is1.52724 in all lenses.

The specification for the optical arrangement is as follows:

Distance d1 from deflected surface to first surface of first scanninglens 7: 64.1 mm;

Thickness d2 at the center of first scanning lens 7: 22.5 mm;

Distance d3 from second surface of first scanning lens to first surfaceof second scanning lens: 76 mm; and

Thickness d4 at the center of second scanning lens: 4.9 mm;

Distance d5 from second surface of second scanning lens to surface to bescanned: 158.6 mm.

The soundproof glass 6 and the dustproof glass 9 having a refractiveindex of 1.514 and a thickness of 1.9 mm are arranged as shown in FIG.1, and the soundproof glass 6 is inclined by 10 degrees with respect toa direction parallel to the horizontal scanning direction in thedeflection surface of revolution.

The F numbers in the vertical scanning direction at the image height inthe most peripheral part and the central part of the scanning opticalsystem are given below:

-   -   Image height 150 mm: 41.8    -   Image height 0 mm: 40.8,    -   Image height-150 mm: 41.1.

FIGS. 11A and 11B are aberration diagrams of the second example asdescribed above. FIG. 11A illustrates the curvature of field, whereX-axis indicates defocusing (mm), and Y-axis indicates image height(mm). The solid line indicates the curvature of field in the verticalscanning direction, and the dotted line indicates the curvature of fieldin the horizontal scanning direction. FIG. 11B illustrates the velocityuniformity, where X-axis indicates percentage, and Y-axis indicatesimage height (mm). The solid line indicates linearity, and the dottedline indicates fθ characteristic.

FIG. 12 is a graph of changes in the radius of curvature in verticalscanning on the first surface (R1) of the first scanning lens (L1) 7,FIG. 13 illustrates that on the first surface (R1) of the secondscanning lens (L2) 8, and FIG. 14 illustrates that on the second surface(R2) of the second scanning lens (L2) 8, and the shapes thereof are asdescribed above.

FIGS. 15A and 15B are graphs of beam spot diameter with respect todefocusing, where FIG. 15A illustrates beam spot diameter in thehorizontal scanning direction, and FIG. 15B illustrates beam spotdiameter in the vertical scanning direction.

In the first and second examples of the optical scanner, the scanningoptical system is formed of two lenses, that is, the first scanning lensand the second scanning lens. However, there may be three or morescanning lenses. In that case, the expected action and operation can beobtained by satisfying the configuration requirements described in thevarious claims.

In the examples, the light source is a semiconductor laser (laser diode(LD)). However, the present invention is also applicable to a multi-beamoptical system using a plurality of semiconductor lasers as the lightsource, and an LD array having a plurality of light emitting points. Ifa resin lens is used as the scanning lens, then the scanning lens havingthe special toric surface or the rotationally symmetric aspheric surfacecan be mass-produced by die forming, thereby reducing cost.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. An optical scanner comprising: a light source; an optical couplerthat couples beams from the light source; an optical line image unitthat forms a line image of the beams from the optical coupler, whereinthe line image is longer in a horizontal scanning direction than in avertical scanning direction; a deflector that deflection-scans the beamsfrom the optical line image unit; and an optical scanning unit thatincludes a plurality of scanning lenses that guide the beams from thedeflector to a surface to be scanned, wherein at least one scanning lenshas a rotationally symmetric aspheric surface and a special toricsurface in which a radius of curvature in a vertical scanning changesfrom an optical axis of the lens surface toward a periphery of thehorizontal scanning direction, and a surface having an area with alargest angle of inclination in an effective range with respect to asurface perpendicular to the optical axis, among the surfaces of allscanning lenses, is the rotationally symmetric aspheric surface.
 2. Theoptical scanner according to claim 1, wherein the surfaces of allscanning lenses in the optical scanning unit are any one surface chosenfrom a group consisting of the rotationally symmetric aspheric surface,and the special toric surface.
 3. The optical scanner according to claim1, wherein among the surfaces of all scanning lenses in the opticalscanning unit, a surface having a largest effective width in thehorizontal scanning direction is the special toric surface.
 4. Theoptical scanner according to claim 1, wherein all the scanning lenses inthe optical scanning unit have an angle of inclination of 30 degrees orless, on the lens surfaces that are in the effective range with respectto the surface perpendicular to the optical axis, all the scanninglenses have the special toric surface in which the radius of curvaturein the vertical scanning changes based on a lens height in thehorizontal scanning direction, and in the special toric surface, avariation in the radius of curvature in the vertical scanning permillimeter of the lens height in the horizontal scanning direction isless than 1.5×10⁻⁴ (1/mm).
 5. The optical scanner according to claim 1,wherein the optical scanning unit includes two scanning lenses, whereinthe beams in a most peripheral part in an effective write width areinclined in a same direction in a deflection surface of revolution withrespect to a normal on a first surface of the two scanning lenses, ascanning lens closest to the deflector has a meniscus shape with aconcave facing the deflector side in the horizontal scanning direction,the first surface of the scanning lens closest to the deflector has anegative power in the vertical scanning direction, and is the specialtoric surface in which the radius of curvature in the vertical scanningchanges from an optical axis of the lens surface toward a periphery ofthe horizontal scanning direction, and the first surface of a scanninglens farthest from the deflector has a positive power in the verticalscanning direction.
 6. An image forming apparatus that forms an image ontransfer paper by executing respective charging, exposure, development,and transfer processes, comprising: an exposing unit that executes theexposure process, including a light source; an optical coupler thatcouples beams from the light source; an optical line image unit thatforms a line image of the beams from the optical coupler, wherein theline image is longer in a horizontal scanning direction than in avertical scanning direction; a deflector that deflection-scans the beamsfrom the optical line image unit; and an optical scanning unit thatincludes a plurality of scanning lenses that guide the beams from thedeflector to a surface to be scanned, wherein at least one scanning lenshas a rotationally symmetric aspheric surface and a special toricsurface in which a radius of curvature in a vertical scanning changesfrom an optical axis of the lens surface toward a periphery of thehorizontal scanning direction, and a surface having an area with alargest angle of inclination in an effective range with respect to asurface perpendicular to the optical axis, among the surfaces of allscanning lenses, is the rotationally symmetric aspheric surface.
 7. Theimage forming apparatus according to claim 6, wherein the surfaces ofall scanning lenses in the optical scanning unit included in theexposing unit, are any one surface chosen from a group consisting of therotationally symmetric aspheric surface, and the special toric surface.8. The image forming apparatus according to claim 6, wherein among thesurfaces of all scanning lenses in the optical scanning unit included inthe exposing unit, a surface having a largest effective width in thehorizontal scanning direction, is the special toric surface.
 9. Theimage forming apparatus according to claim 6, wherein all the scanninglenses in the optical scanning unit included in the exposing unit havean angle of inclination of 30 degrees or less, on the lens surfaces thatare in the effective range with respect to the surface perpendicular tothe optical axis, all the scanning lenses have the special toric surfacein which the radius of curvature in the vertical scanning changes basedon a lens height in the horizontal scanning direction, and in thespecial toric surface, a variation in the radius of curvature in thevertical scanning per 1 mm of the lens height in the horizontal scanningdirection is less than 1.5×10⁻⁴ (1/mm).
 10. The image forming apparatusaccording to claim 6, wherein the optical scanning unit included in theexposing unit includes two scanning lenses, wherein the beams in a mostperipheral part in an effective write width are inclined in a samedirection in a deflection surface of revolution with respect to a normalon a first surface of the two scanning lenses, a scanning lens closestto the deflector has a meniscus shape with a concave facing thedeflector side in the horizontal scanning direction, the first surfaceof the scanning lens closest to the deflector has a negative power inthe vertical scanning direction, and is the special toric surface inwhich the radius of curvature in the vertical scanning changes from anoptical axis of the lens surface toward a periphery of the horizontalscanning direction, and the first surface of a scanning lens farthestfrom the deflector has a positive power in the vertical scanningdirection.